The point of departure for this invention is a rain detector of the type which includes a sensor for a windshield and means for comparing the level of a signal emitted by the sensor to a threshold, with the means for comparing emitting a switching signal to a windshield wiper unit. German Patent DE 3,314,770 discloses a rain detector of this type that emits a signal for controlling a windshield wiper system. The signal emitted by the sensor is a measure of the water or dirt on the wetted windshield which might have to be cleaned.
The rain detector is based on an optical measuring method. Radiation is coupled from a radiation source through a prism into the windshield and hits the surface of the windshield which requires cleaning. The radiation is reflected at a defined angle from the exterior surface of the windshield that is wetted with water or soiled, and is further conducted through the windshield and coupled out of the windshield through the prism. The reflected radiation is detected by a radiation sensor, which gives a signal to an evaluation unit that is connected to its output. The radiation sensor detects only those radiation components which are emitted by the radiation source and reflected on the surface of the windshield at the angle producing total reflection. During undisturbed total reflection, the radiation sensor gives off a signal at a certain level or quiescent level. This level corresponds to a resting level. If there are changes in the intensity of the totally reflected radiation caused by water drops or dirt particles, the level of the emitted signal changes and is compared with a threshold. If the threshold is exceeded, the switch-on signal for a windshield wiper system is emitted.
The threshold, with which the signal emitted from the sensor is compared, is automatically adjusted as a function of the extreme value of the signal level. This adjustment of the threshold compensates for long-term drift and temperature drift of the electronic circuit and takes into consideration the condition of the windshield surface, which changes constantly, for example, due to scratches. The adjustment range is limited to the input voltage range that an amplifier which is connected as a comparator is able to process.
In addition to long-term and temperature drift, the resting level of the signal emitted by the sensor depends notably on the structural conditions for the installation of the rain detector on the windshield. In addition to manufacturing tolerances during the production of electronic and optical components, different windshield thicknesses, different spectral transmission coefficients of the windshields and, particularly, the technique of connecting the rain detector to the windshield must be taken into consideration.